Blank part. on my HDD, can I install a guest OS to it?

[Starks]

That is, can I use VirtualBox to install Windows 7 on a blank partition, sidestepping the need to install from DVD?

[Sasquatch]

If you have a full partition that is not used, you can install it on the drive itself for your own hardware, creating a dual boot situation. But if you don't like that, you can either use RAW Disk Access, which is actually not advised to use, or you can create a virtual hard drive image and store that on the partition. Make sure that you make a large enough base file, so it can grow enough. It's no good to create a 10 GB drive if Win 7 will already use 8 of it after a clean install. The initial file of the VDI will be a few KB, but once you start to use it and install an OS, it will grow dynamically. There is only one limit to how big it gets, and that's the limit you set when creating it. See the manual for more information.